Digital holography imaging has been used to simplify setups, for achieving a large field of view and eventually to avoid having a lens that may be expensive, sensitive to vibrations and has a need for alignment and re-alignment with the rest of the setup. Coherent light for illumination is traditionally used in digital holography imaging to get retraceable interference from the object or substance that is to be imaged. Light is diffracted by the illuminated object or substance and interferes with the reference light from the illumination source. This interference signal is captured by an image sensor. The sampled signal will not show an apparent image. The sampled signal, however, contains holographic information that may be used to numerically reconstruct an image plane corresponding to an image (in focus) produced with a lens system. The coherent light source may be naturally coherent as e.g. a laser or an incoherent light source combined with an aperture as e.g. an LED with a pin hole.
Digital holography may be performed with a lens for focusing purposes or done lens free. Lens free imaging has been used in biological applications for counting blood cells. A flow cytometer has traditionally been used as the standard diagnostics device for tests that rely on counting different types of blood cells. In a flow cytometer, a sample flows through a detection zone, typically one cell at a time, and cell properties are determined by an illumination and detection system. Using lens free imaging, a cheaper and more compact solution is achieved, suitable for lab on chip systems.
In one proposed system, a suspension with blood cells is guided through a flat channel on a chip. A light source illuminates the flow above the channel and an image sensor collect light underneath the channel. A simplified lens-free imaging technique without holographic reconstruction uses only the shadows of the cells passing the channel. This approach produces low quality image information compared to a holographic approach.
An alternative to this proposed solution would be to use a similar setup but with a traditional lens system. Focused images would then be obtained with more information than is available in the shadow images.
Other solutions are however known where digital holography imaging has been performed using holographic reconstruction to present an image corresponding to a focused image using a lens imaging system. These solutions are usually small and implemented on lab-on-chip systems. A problem with such systems has traditionally been that with a planar incident wave setup, the resolution in the reconstruction is insufficient to resolve the internal structure of the cells, making cell differential analysis a very challenging problem.
Another proposed system for counting particles in a suspension using digital holography imaging with a holographic approachuses a fiber optic cable as light source to resemble a light source with an aperture.